Carbon Dioxide Plasma as a Versatile Medium for Purification and

Article pubs.acs.org/JPCC

Carbon Dioxide Plasma as a Versatile Medium for Purification and Functionalization of Vertically Aligned Carbon Nanotubes Deepu J. Babu,† Sandeep Yadav,† Thorsten Heinlein,† Gennady Cherkashinin,‡ and Jörg J. Schneider*,† †

Fachbereich Chemie, Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Alarich-Weiss-Strasse 12, and ‡Fachbereich Material und Geowissenschaften, Fachgebiet Oberflächenforschung, Jovanka-Bontschits-Strasse 2, Technische Universität Darmstadt, 64287 Darmstadt, Germany S Supporting Information *

ABSTRACT: Three-dimensional (3D) architectures obtained by the structural assembly of 1D nanomaterials are regarded as the next generation building blocks for sensors, electronics, photonics, and bioelectronic applications. Purification and functionalization of such 3D ordered structures are crucial for realizing their full potential. Plasma functionalization, compared to any solution based process, is favorable in retaining the alignment while functionalizing such structures. However, the commonly employed plasma processes like O2 or Ar plasma can be highly detrimental to well-aligned ordered nanostructures and thus might affect the properties intimately associated with their 3D structure. Here, for the first time, we investigate the mild nature of a radio frequency CO2 gas plasma as an effective source for purification and functionalization of vertically aligned CNT structures and study the effects of this functionalization onto the purification and functionalization by physical and chemical techniques (HRTEM, XPS, Raman). We found that CO2 plasma selectively etches the amorphous carbon present in the vertically aligned CNT structure. Moreover, it is as effective as the widely used but more aggressive O2 plasma in functionalizing the CNT. Unlike an O2 or Ar plasma, CO2 plasma has the tremendous advantage of retaining the structural integrity of the CNT structures.

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INTRODUCTION The assembly of one-dimensional nanostructures into wellaligned three-dimensional (3D) architectures has opened up a plethora of new possibilities in the fields of gas sensing, electronics, and photonics.1,2 The ability to modify and tailor these 3D materials without compromising their characteristic well-defined alignment and structural integrity is central to the very concept of nanomicromacro integration. Among the different 3D nanomaterials, vertically aligned carbon nanotubes (VACNT) are one of the most promising and widely studied materials. Their good electrical conductivity, high mechanical strength, and large specific surface area3 find a wide range of applications in the field of energy storage,4,5 optics,6 field emission,7 and gas sensing/adsorption.8,9 Vertically aligned CNT structures are generally obtained via in situ methods like plasma enhanced CVD10,11 or thermal CVD.3,12−15 The alignment of the CNT is achieved during their growth phase and is due to the close proximity effect arising from van der Waals interaction between neighboring, uprising CNT.16 The © 2014 American Chemical Society

as-synthesized CNT structures are generally associated with the presence of minor amounts of carbonaceous impurities, which are in many cases detrimental for subsequent follow-up applications.17 Effective removal of these amorphous carbon phases without destroying the 3D alignment is crucial in realizing the full potential of such VACNT architectures. Liquid phase purification methods employing harsh acids, oxidants, or a mixture of both,17−19 which are routinely used for flat lying, randomly distributed CNT cannot be used for the removal of such residual amorphous carbon from VACNT. This is because the strong capillary forces imposed by the penetrating solvents, collapse the individual CNT within the array and cause them to cling together thereby destroying their unique vertical alignment.20−22 Therefore, gas phase methods are generally preferred over liquid phase purification processes. The Received: March 19, 2014 Revised: May 5, 2014 Published: May 8, 2014 12028

dx.doi.org/10.1021/jp5027515 | J. Phys. Chem. C 2014, 118, 12028−12034

The Journal of Physical Chemistry C

Article

evaporation followed by sputtering of 1.2 nm Fe layer which together with the Al layer acted as the catalyst for the CNT growth.15 CVD synthesis was carried out in the presence of parts per million quantities of water at 850 °C for 15 min with ethylene as the carbon source. CO2 plasma functionalization was carried out in a capacitively coupled radio frequency plasma apparatus (Diener, model Femto, Germany, 13.56 MHz, maximum RF power limited to 200 W). The sample was degassed to pressures less than 0.2 mbar for 5 min followed by the introduction of CO2 gas. The RF generator was switched on only after achieving a stable pressure at the set flow rate. Characterization. Raman spectroscopy was carried out using a LabRAM high resolution microscope (Horiba Jobin Yvon, model HR 800). The excitation source was 514.5 nm Ar laser. SEM measurements were performed on a Philips XL30 FEG. TEM investigations were carried out on a FEI Tecnais F20 G2 operated at 200 kV. CNTs were dispersed in ethanol by ultrasonification for 10 min before a few drops of the dispersion were placed on a lacey carbon grid. XPS measurements were performed on a PHI 5000 (Physical Electronics) spectrometer with monochromatic AlKα (hν = 1486.7 eV) source. The photoelectron spectra were collected at a pass energy of Epass = 23.5 eV with an electron escape angle of 45°. All binding energies are referred to the Fermi level of an Ag foil. The background of the XPS spectra was subtracted using a Shirley type function. The photoelectron peak positions and area were obtained by weighted least-squares fitting of model curves (70% Gaussian, 30% Lorentzian with a Doniach−Sunjic high binding energy tail) to the experimental data.

difference in oxidation rates of amorphous carbon and CNT can be successfully exploited in a gas phase purification process. Amorphous carbons are more reactive due to the presence of high defect density.17 Thermal purification in air at 700 °C represents a straightforward purification route to CNT, also exemplifying their extraordinary thermal stability compared to other carbon allotropes.23 The harsh thermal conditions lead to a considerable amount of weight loss due to the degradation of the CNT under these conditions. Thus, an adherence to a narrow thermal operating window has to be maintained which needs extensive experimental optimization and restricts its applicability. Purification of CNT with steam and H2 has been reported.24,25 Recently, plasma treatment of CNT has attracted a great deal of interest due to the rapidity of the process and range of functional moieties that could be grafted onto the surface of VACNT without altering the CNT alignment.22,26 O2 plasma functionalization is one of the most widely used plasma techniques for functionalization of CNT. It not only attacks the amorphous carbon but also grafts hydroxyl, carbonyl, and carboxyl groups to the CNT surface.26,27 However, O2 plasma is highly aggressive and may consume the CNT completely within a few minutes. In addition, even few seconds of exposure to O2 plasma is able to completely alter the surface chemistry of CNT and can turn a native superhydrophobic surface to a superhydrophilic surface.27,28 Moreover, the large density of defects induced by O2 plasma surface reactions typically deteriorates the CNT structure. With Ar and N2 plasma, chemical effects are known to proceed by an extensive etching of the graphitic CNT structure which creates defects as holes and voids in individual CNT. Though CO2 plasma is widely used in biology29,30 and for polymer functionalization,31−33 surprisingly few reports are available in other fields. One such report is the dielectric barrier discharge CO2 plasma diluted with He, Ar, or N2 used for functionalizing CNT.34 It was found that the diluent gas plays a crucial role in the overall plasma reaction, and it seriously alters the extent of functionalization. Several reports suggest the decomposition of CO2 in plasma to produce coke and CO.35,36 So far the absence of research reports on CO2 plasma treatment of CNT structures might be attributed to the assumption that decomposition of pure CO2 without any diluent gas results in the formation of coke on the nanotube surface.34 However, this assumption is not substantiated if one takes into account the endothermic equilibrium condition in the Boudouard reaction, and this prompted our research described herein. In the present study, we investigate, for the first time, the effect of pure CO2 plasma on VACNT architectures. Contrary to current knowledge, it is observed that CO2 plasma resulted in the selective purification and functionalization of VACNT architectures. Unlike all other plasmas reported so far, the defect concentration of the CNT was found to decrease with CO2 plasma exposure time. The CNTs were thus purified and functionalized keeping the highly aligned CNT architecture fully intact. We feel that the reported process of purification and mild chemical functionalization is highly advantageous for future studies toward the applicability of 3D nanomaterials.

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RESULTS AND DISCUSSION The WACVD synthesis carried out for 15 min resulted in ultralong vertically aligned CNT with an average height of ∼800 μm as shown in Figure 1a. High magnification SEM image reveals the dense and highly aligned nature of the CNT structure. The CNTs are usually double or up to maximum four walled with an average inner diameter of 8 nm. Following their synthesis, we performed a high temperature treatment under CO2 atmosphere to purify the vertically aligned CNT37 further. Experiments were carried out for a range of temperatures between 600 and 750 °C. An S shaped curve, typical for the Boudouard reaction, with the start of exponential rise of weight loss at around 625 °C was obtained (see Supporting Information Figure S1a). At temperatures of 700 °C, the complete CNT arrays were consumed. It was also found that the history of the CNT synthesis, in particular the extent of postsynthesis hydrogen treatment, affected the onset of the weight loss. Postsynthesis hydrogen treatment weakens the adhesion between the catalyst particles and the CNT,38 thus exposing the iron catalyst particles to the CO2 atmosphere thereby shifting the equilibrium.39 The post-treatment procedure is typically used to detach the VACNT from the growth substrate, e.g., for future application purposes. TEM investigation of the sample revealed excessive amounts of amorphous carbon (see Supporting Information Figure S1b) even for samples treated at 650 °C. This points to the very narrow operating window (see Supporting Information Figure S1a) which is well in accord with what would be expected from the endothermic Boudouard reaction. Such a steep slope was also reported for air oxidation purification of CNT.23 This in fact now opens up the possibility to tune and adjust the plasma process parameters to optimize the purification and functionalization process. However, compared to the thermal treatment,

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EXPERIMENTAL METHODS Sample Preparation. Vertically aligned CNTs were synthesized by water assisted chemical vapor deposition (WACVD) process. A buffer layer of Al of 10−12 nm thickness was deposited on a boron doped Si/SiO2 wafer by thermal 12029



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Figure 1. (a) SEM image of as-prepared vertically aligned CNT structure (scale bar = 500 μm). The higher magnification image in the inset (scale bar = 5 μm) shows the vertical alignment. (b) TEM of the as-prepared CNT obtained after dispersing in ethanol and unhinging them from the VACNT arrays by ultrasonification (scale bar = 50 nm). (c) Raman spectra of CO2 plasma treated CNT at different exposure times. (d) Plot of ID/IG and I2D/IG against time of CNT samples treated by CO2 plasma.

NH3,49 CF4,50,51 H2,52 or H2O53 lead to an increase in the defect ratio. This is in sharp contrast to the CO2 plasma treatments reported herein. To the best of our knowledge, the observed considerable decrease in the defect concentration upon plasma exposure is thus unique for the CO2 plasma procedure. The I2D/IG ratio of the CO2 plasma treated samples shown in Figure 1d displays an increasing trend until a processing time of 4 min. The observed increase of the I2D/IG ratio is the result of the removal of the disordered amorphous carbon and is associated with an increase in the crystallinity of the so treated VACNT. This clearly underscores the mild and purifying nature of the CO2 plasma when compared to the so far widely used other plasma gases, e.g., mainly O2. Exposure times greater than 4 min lead to an increase in the defect concentration reaching a maximum at 8 min and then decreasing again. At 10 min of CO2 plasma exposure, an ID/ IG value of 0.63 was obtained. At longer exposure times (t > 4 min), the CO2 plasma under the given conditions is found to slowly start to damage the CNT structure as revealed by a slight increase in the defect concentration. This assumption is further validated by the decrease in the crystallinity as shown in Figure 1d. Degradation of the structure leads to the formation of more amorphous carbon, subsequently purified upon longer exposure times. It is thus concluded that this fact has led to the observed decrease in the defect concentration at 10 min. Experiments carried out for still longer times (10 min up to 20 min) displayed the same trend of increase and then decrease of the defect concentration. From our previous XPS studies27 it is known that the amount of oxygen present in the as-prepared VACNT architectures is negligible. On the other hand, CNT samples exposed to CO2 plasma displayed a considerable amount of oxygen as seen from the survey spectra in Figure 2a. A more detailed analysis of the nature of functionalization was carried out by deconvoluting the

plasma treatment of the CNT offers distinct advantages of better flexibility and tunability without compromising the alignment of CNT. In our previous experiments with O2 plasma,27 it was seen that, though an O2 RF plasma was highly efficient in functionalizing the CNT, the process parameters were difficult to optimize. The defect concentration as measured by Raman increases and reached a constant value in less than 60 s. Moreover CNT burn off was observed for power >60 W and exposure times >4 min. In the present study, carbon dioxide gas was chosen for plasma studies because of its mild oxidizing nature compared to widely studied oxygen gas. Raman spectroscopy is used as the primary tool for investigating the effect of different plasma parameters on CNT due to its nondestructive nature. The D-band centered at ∼1350 cm−1 is assigned to the presence of disorder in the graphitic material while the G-band (∼1590 cm−1), corresponding to the tangential vibration of the carbon atoms, is a meaningful indicator for the graphitization of the CNT. The ratio between the D-band and G-band can thus be used as a tool for quantifying the relative purity of the CNT sample. The peak at ∼2690 cm−1 is assigned to the second-order harmonic of the D-band.40−42 The second-order 2D band is not the result of an elastic defect related scattering process and is thus observable for defect-free sp2 carbons and should give a significant fingerprint for the integrity of the structure.40 Hence the ratio of I2D/IG may be taken as a measure of the crystallinity of the CNT.43 In the first set of experiments, the effect of plasma exposure time on CNT was studied. Keeping the power constant at 60 W and chamber pressure at 0.52 mbar, plasma exposure times were varied from 0 up to 10 min (Figure 1c). As shown in Figure 1d, the defect ratio as given by ID/IG decreased continuously with CO2 plasma exposure for time scales up to 4 min. It is noteworthy that commonly employed plasma treatment procedures using O2,27,44,45 N2,46,47 air,48 Ar,46 12030



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Figure 2. (a) XPS survey spectra of as-synthesized and CO2 plasma treated CNTs exposed to different plasma treatment times. High resolution photoelectron spectra of (b) 4 min CO2 plasma treated and (c) 10 min CO2 plasma treated CNT. (d) TEM image of CNT subjected to 10 min of CO2 plasma treatment (scale bar = 50 nm).

O* radicals would attack, in the herein described 3D aligned CNT architectures, the amorphous carbon as well as the graphitic walls of the individual CNTs. However, the very high density of O* radicals in an oxygen plasma leads to considerable structural deterioration of CNT (as can be observed by the increase in the ID/IG ratio in Raman). In our experiments with O2 plasma, which were carried out even for a low energy input of 10% of the total power, 0.4 mbar chamber pressure, and for time less than 30 s, still a considerable increase in the defect concentration was observed. The absence of any reports whatsoever on a considerable decrease of the defect ratio by an O2 plasma treatment furthermore underscores our assumption that a preferential removal of amorphous carbon is yet not possible by just a fine-tuning of the plasma parameters generating even mild O2 plasma conditions. The dilution of the O2 gas seems also to be not an alternative here as the presence of diluent gases like Ar, He, or N2 further aggravate the deterioration of a material by generating even more O* radicals within the reaction chamber36,44 due to the reaction ArM + O2 → Ar + O* + O*. In contrast, in a CO2 plasma, a large fraction of O* radicals generated are already quenched by the reaction56 CO2 + O* → CO + O2. This assumption is further supported by the fact that CO2 gas is mainly transformed to CO and O gases in a plasma process.56−58 The lower density of O* radicals present in a CO2 plasma thus leads to a preferential removal of amorphous highly unordered carbon compared to the more stable graphitic carbon structure units present in the CNTs. This finally leads to the observed purification effect. Once a large portion of amorphous carbon is removed along that route, CNT deterioration can accelerate as was observed in the increase

C 1s high resolution photoelectron spectrum of the 4 min CO2 plasma functionalized CNT (Figure 2b). Compared to the asprepared CNT, CO2 plasma functionalization for 4 min resulted in the appearance of a new peak at around 286.6 eV (Figure 2b). The C 1s photoelectron spectra of the treated sample was fitted by four components, CC at 284.1 eV, C−C at 285 eV, C−O at 286.6 e,V and O−CO at 288.6 eV. This spectrum is remarkably similar to the C 1s spectrum of O2 plasma functionalized samples.27 Even after exposing to 10 min of CO2 plasma treatment, the concentration of the functional groups remains unchanged (see Figure 2c). XPS quantitative analysis of VACNT samples subjected to 4 min of CO2 plasma treatment indicated ∼37.5 atomic percentage of oxygen moieties tethered to the CNT surface. VACNT exposed to 8 and 10 min of CO2 plasma also showed nearly the same atomic concentration (∼37.5%) of oxygen (see Figure 2a). Thus, it has to be assumed that upon CO2 plasma functionalization, oxygen concentration on CNT increases and can reach saturation in 4 min or less. Saturation of oxygen concentration at ∼38% by plasma treatment have been previously reported.27,45 From the Raman and XPS studies, it can thus be deduced that the CO2 plasma is indeed as efficient as O2 plasma in functionalizing the CNT, but importantly and unlike the O2 plasma it does not structurally deteriorate the VACNT architecture. Why this is so? Typically a number of different reactive species like neutral molecules, ions, free radicals, etc. are generated in a plasma process. However, the overwhelming number of chemical reactions in a plasma process are based on free radical chemistry processes.54 For example, in oxygen plasma, many authors26,44,45,55 have pointed out that oxygen radicals (O*) are the most prominent and reactive ones. These 12031



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Figure 3. (a) SEM image of VACNT structure after 10 min of plasma treatment (scale bar = 250 μm). Inset shows (scale bar = 5 μm) that the vertical alignment is still retained after 10 min of plasma treatment (compare to Figure 1a). (b) Raman spectra of CNT treated with CO2 plasma at various RF-power conditions. (c) Variation of ID/IG and I2D/IG with respect to CO2 plasma power. (d) Variation of defect concentration with plasma chamber pressure.

of the ID/IG ratio, after having reached a minimum at 4 min (see Figure 1d). In the absence of any direct plasma characterization process, this proposed mechanism explains in an uncontroversial way the observed experimental results. Direct evidence can only be obtained by an in situ and quantitative observation of the plasma species involved in the process. However, this is currently beyond the scope of the studies. The purifying nature of the CO2 plasma was further substantiated with TEM measurements shown in Figure 2d, which shows the purified CNT structure obtained at 10 min of CO2 plasma exposure. Compared to the as-synthesized structure, the presence of amorphous carbon is significantly reduced in the treated sample. SEM images of the CNT structures (Figure 3a) reveal that the vertical alignment is still intact even after 10 min of plasma exposure. The top surface of the CNT displayed some morphological changes as the tips of the nanotubes agglomerate in irregular however periodic patterns (see Supporting Information Figure S2). This specific morphological formation is a known phenomenon under plasma treatment conditions which occurs on the surface of such ultralong and highly parallel CNT structures.45,46,55 The effect of other plasma parameters on the functionalization and arrangement of VACNT architectures were also explored. For an exposure time of 2 min and chamber pressure of 0.5 mbar, the plasma power was varied from 30 to 120 W. At still lower powers (120 W) destruction of the VACNT sample was observed. The Raman spectra of the CNT samples treated at various powers are shown in Figure 3b. The decrease of the intensity of the 2D band and the increase in the

intensity of D-band with increase in plasma power is evident from the Raman spectra. To obtain an even clearer picture, the ratios of ID/IG and I2D/IG against power are plotted in Figure 3c. An increase of defect concentration (ID/IG) with an increase in power is a typical observation under plasma treatment, irrespective of the plasma gases used. An increase of power accelerates both physical and chemical effects of the employed plasma resulting in the generation of more active species in the plasma,45,59 which can destructively attack the CNT walls. This is reflected in an increase in the ID/IG ratio as well as a decrease in the I2D/IG ratio. The effect of chamber pressure was also studied by keeping the power constant at 60 W for an exposure time of 2 min. It was seen that the defect concentration decreased with an increase in chamber pressure until 0.5 mbar, thereafter showing an increasing trend. High chamber pressure, similar to high power, leads to an increase in the plasma density.59,60 This occurrence explains the increase of the defect concentration for chamber pressures >0.5 mbar. When O2 and CO2 plasmas are compared under similar operating conditions, the weight loss observed for CO2 plasma is negligible, emphasizing again its mild nature. This nonaggressive nature of CO2 plasma might thus be advantageous when dealing with other 3D aligned nanostructures as well as thin films (e.g., graphene), which also requires mild oxidizing conditions.

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CONCLUSIONS In conclusion, the mild nature of CO2 plasma as a medium for efficient purification and functionalization of 3D nanomaterials was investigated. The effect of different CO2 plasma parameters 12032



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on 3D vertically aligned CNT arrays was systematically studied, and it was found that CO2 plasma is as effective as O2 plasma in functionalizing the CNT. However, CO2 plasma offers the important additional advantage of more flexible operating conditions with minimal structural damages to VACNT architectures. Under CO2 plasma conditions, a selective etching of amorphous carbon on CNT is possible which allows one to purify VACNT array structures without being detrimental to their morphology. On a broader perspective, the chemically mild conditions and nonaggressive nature of the RF-generated CO2 plasma may certainly prove beneficial when selective and highly controllable functionalization of 3D nanomaterials is required. Currently we are exploring this route in more depth.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details regarding the CO2 thermal treatment along with its TEM image; SEM images of the top surface of the CNT after plasma treatment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax +49 6151 163224/6151 16-3470; e-mail joerg. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported within the priority program SPP 1570 of the Deutsche Forschungsgemeinschaft, DFG. TEM measurements (J. Engstler, TU Darmstadt) were done at the ErnstRuska-Institute (ERC) Jülich under contract ERC-TUD1. We are grateful to Prof. Ralf Riedel (TU Darmstadt) for providing access to the micro Raman measurement setup. We acknowledge additional financial support through the LOEWE Initiative STT (Sensors towards Terahertz) funded by the State of Hesse.

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REFERENCES

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